1. Field of the Invention
The present invention relates to erbium doped fiber amplifiers. In particular, the invention relates to an arrangement of fiber amplifier segments with spectral equalizers between segments to provide extended bandwidth.
2. Description Of Related Art
Undersea communication network systems require repeaters periodically spaced to compensate for attenuation in the signal transmission medium. Optical fiber networks include repeaters connected between links of optical fiber cable. The optical fiber cables include one or more optical fibers and often include conductive wires (e.g., copper wires) to deliver power to the repeaters.
The repeaters are amplifiers. Older (regenerative) repeaters convert the input optical signal into an electrical signal which is amplified and then converted back to an output optical signal. However, most modern optical repeaters in lightwave transmission systems use optical amplifiers, and erbium doped fiber amplifiers (EDFA) have become the dominant technique for optical signal amplification. An EDFA includes a length of erbium doped fiber in which a lightwave propagates. An energy source or pump laser (e.g., an InGaAsP capped mesa buried heterostructure device) xe2x80x9cpumpsxe2x80x9d the length of erbium doped fiber into an xe2x80x9cexcitedxe2x80x9d state. From this state, two consequences follow. First, signal gain is produced through emission stimulated by signal light passing through the xe2x80x9cexcitedxe2x80x9d fiber. Second, optical noise is produced through spontaneous decay of the xe2x80x9cexcitedxe2x80x9d state of the length of erbium doped fiber. The design of the fiber amplifier seeks to maximize the signal gain and minimize the optical noise produced while meeting the required power output and bandwidth specification.
It is desired to provide a wide band fiber amplifier to carry more information than can be carried by a narrow band amplifier. Known EDFAs have a bandwidth limit of about 40 to 45 nanometers over which the desired design specification and noise figure can be achieved. However, to achieve a wide band amplifier in excess of 40 to 45 nm, known wide band amplifiers have been realized by splitting the signal into two wavelength ranges. For example, the signal spectrum may be split into a first channel carrying a signal in a band from 1530-1560 nm and a second channel carrying a signal in a band from 1570-1610 nm. The first and second channels are amplified in respective EDFAs and then recombined on the amplifier""s output fiber.
One known approach to achieve the band splitting and band recombination uses circulators together with extended band reflective filters. Another known approach to achieve the band splitting and band recombination uses wavelength division multiplexers. Using both techniques, but especially with the circulator design, the amplifier is limited by insertion loss problems, and the addition of a common stage of amplification is required in both first and second channels which adds to the complexity of the design and gain balancing between the channels. Using both techniques, the amplifier is limited by isolation problems. Multi-path interference generated by reflections from a point source or distributed sources arises when there is insufficient isolation between the amplifier components. Band splitting amplifiers also waste potential bandwidth in the guard band between the two channels that could otherwise be used to carry signals.
It is an object to the present invention to provide a wide band optical amplifier without a need for wavelength splitting equipment. It is another object to distribute the spectral gain equalization function between multiple erbium doped fiber amplifier stages.
These and other objects are achieved in an optical amplifier that includes first, second and third amplifier stages, a first equalizer coupled between the first and second amplifier stages, a second equalizer coupled between the second and third amplifier stages, and a third equalizer coupled between an amplifier output and the third amplifier stage. In another embodiment, the first stage includes a length of first erbium doped fiber coupled between an input and the first equalizer. The second stage includes a second erbium doped fiber coupled between the first equalizer and the second equalizer. The third stage includes a third erbium doped fiber coupled between the second equalizer and the third equalizer.